The electrophotographic image-forming methods currently used in copying, printing, etc. include a two-component development method in which a magnetic carrier and a toner as a colorant are used in combination, and a one-component development method which uses a toner which itself is magnetic.
Because of the nonuse of a carrier, the one-component development method has many advantages, for example, that the development apparatus is simple (the sizes of the development apparatuses are about a half to one-third the sizes of those used in the two-component development method), and that the management of developers is easy. However, when this method is used for forming color images, darkish magnetic toners should be used and images of vivid colors cannot be obtained.
The reason for this is as follows. In order to obtain clear color images by the one-component development method, magnetic toners themselves should be colored vividly. However, since the magnetic material particles serving as the bases thereof are generally black, merely forming a colored film directly on the surface of such base particles results in a dark color as a whole.
Accordingly, the two-component development method is employed at present for forming color images. However, since color copying necessitates four colors, i.e., three primary colors and black, a larger development apparatus is necessary as a matter of course.
Additionally, there are problems concerning the management of developers, the treatment of carriers resulting from development, etc.
Consequently, if vivid colors are obtainable by the one-component development method, the use of this method is preferred because the copier is simple and compact and the problems concerning the management of developers and the treatment of carries are eliminated. However, magnetic toners for the one-component development method which are suitable for the formation of color images have not been obtained so far.
Under these circumstances, the present inventors previously proposed: a method which comprises dispersing a base particle into a metal alkoxide solution and hydrolyzing the metal alkoxide to thereby form on the surface of the base particle a metal oxide film having a uniform thickness of 0.01 to 20 .mu.m (Unexamined Published Japanese Patent Application No. 6-228604); a functional powder having thereon plural layers of a metal oxide thin film and a metal thin film alternatively (Unexamined Published Japanese Patent Application No. 7-90310); and a process comprising heating a powder coated with a multilayered metal oxide film to thereby produce a powder having a multilayered metal oxide film which is denser and more stable (WO96/28169).
The above-described powder having plural layers of a metal oxide film or a metal film can be made to have a special function by regulating the thickness of each film. For example, when coating films having different refractive indexes are formed on the surface of the base particle in a thickness corresponding to one-fourth the wavelength of an incident light, then a powder which reflects all of the incident light is obtained. This suggests the possibilities that by applying the above technique to the base particle of a magnetic material, a magnetic powder for magnetic toners might be produced which wholly reflects light and has a vivid white color, and that further forming a colored layer on the surface of this magnetic powder and then forming a resin layer thereon might yield a color magnetic toner colored vividly.
Accordingly, an object of the present invention is to further develop the above-described techniques proposed by the present inventors to thereby provide a color magnetic toner with which a vivid color is obtained even by the one-component development method.